Audio coding is used in many applications and environments such as satellite radio, digital radio, internet streaming (web radio), digital music players, and a variety of mobile multimedia applications. There are many audio coding standards, such as standards according to the motion pictures expert group (MPEG), windows media audio (WMA), and standards by Dolby Laboratories, Inc. Many audio coding standards continue to emerge, including the MP3 standard and successors to the MP3 standard, such as the advanced audio coding (AAC) standard used in “iPod” devices sold by Apple Computer, Inc. Audio coding standards generally seek to achieve low bitrate, high quality audio coding using compression techniques. Some audio coding is “loss-less”, meaning that the coding does not degrade the audio signal, while other audio coding may introduce some loss in order to achieve additional compression.
In many applications, audio coding is used with video coding in order to provide multi-media content for applications such as video telephony (VT) or streaming video. Video coding standards according to the MPEG, for example, often use audio and video coding. The MPEG standards currently include MPEG-1, MPEG-2 and MPEG-4, but other standards will likely emerge. Other exemplary video standards include the International Telecommunications Union (ITU) H.263 standards, ITU H.264 standards, QuickTime™ technology developed by Apple Computer Inc., Video for Windows™ developed by Microsoft Corporation, Indeo™ developed by Intel Corporation, RealVideo™ from RealNetworks, Inc., and Cinepak™ developed by SuperMac, Inc. Some audio and video standards are open source, while others remain proprietary. Many other audio and video coding standards will continue to emerge and evolve.
Stereo coding refers to audio coding that uses two different coding channels. Since humans have two ears, which can detect sound in a directional fashion, stereo coding can improve the quality of perceived sound that is detected by humans from a decoded audio signal. One relatively intuitive way to encode stereo sound information is to use left and right channels to code left and right signals respectively. However, for complex sound such as musical recordings, left and right encoding channels do not work particularly well, due to the fact that left and right microphones can detect the same sounds at different time delays. For this reason, mid-side (MS) stereo was developed to improve stereo sound quality.
In MS stereo, two different signals are used to respectively code the location of sound sources and the reverberant ambiance of the stereo image. Basically, MS stereo is encoded via a middle signal and a side signal. Following decode, a left channel is generally the sum of two coded channels, while the right channel is generally the difference between the two coded channels. By using a middle stereo component and a side stereo component, stereo redundancies in left and right channels can be exploited to achieve better quality stereo coding, as well as better levels of compression for a given level of quality.